What Do You Mean 11 inches of Water Column?

Every once in awhile I’ll get asked a question about the LP gas operating pressure for RV gas appliances, or about checking the LP gas system for leaks. I usually explain that you should have a LP gas operating pressure test and a leak test performed by an authorized RV service center at least annually, and anytime the LP gas system is worked on. The reason I recommend this is because RV service centers have trained technicians with the proper equipment to perform these tests.

Most RVers don’t purchase test equipment like a manometer, used to perform LP gas pressure and leak tests. The cost of some test equipment makes it prohibitive, and the thought of only using it on an annual basis makes buying it kind of pointless. But if you did in fact have a manometer the process of testing the LP gas system is not that difficult.

Note: If you are not comfortable working around LP gas, or don’t understand how to properly use LP gas test equipment have all testing done by an authorized RV service center.

What does 11 inches of water column mean?

There are several different types of manometers available for testing the RV LP gas system pressure, but the most accurate type is what is referred to as a U tube water column manometer. A water column manometer measures the LP gas operating pressure in units of water column inches. You add water to the manometer until the water on both sides of the U shaped tube register evenly at zero on the scale. When you apply the LP gas pressure to the manometer, (usually by removing and connecting the hose to one of the range top burners and turning the burner gas supply on), the gas pressure will force the water down the tube on one side and up the tube on the other side.

Note: Light one of the other range burners to ensure the LP gas is turned on and there is a steady flow of gas to the range top.

Now you combine the measurements, in half inch increments, that the water dropped from zero on one side of the tube and raised from zero on the other side. This total measurement represents the inches of water column pressure. For RV appliances to operate properly, and safely the operating pressure should be 10 ½ to 11 inches of water column.

On an RV the LP gas pressure is regulated by a two stage regulator. The job of the first stage is to regulate the vapor pressure coming out of the container, which can be as high as 250 psi, down to 10 psi or less. The second stage takes that already lowered vapor pressure and regulates it down to the 11 inches of water column operating pressure. The reason for using a manometer to test this pressure is that the operating pressure for LP gas appliances is extremely low if measured in pounds per square inch; somewhere in the neighborhood of .5 psi.

If the LP gas operating pressure is too high or too low you can adjust it at the RV regulator. To do this remove the plastic cap on the front of the regulator and you will see the adjusting screw. If the water column pressure is less than 11 inches you can increase it by turning the adjustment screw clockwise. If it’s more than 11 inches you can decrease it by turning the adjustment screw counterclockwise. Always turn the adjustment screw slowly, and make sure a range burner is lit when making adjustments to the regulator. Adjust the regulator until the readings on both sides of the U tube manometer equal 11 inches of water column.

Happy RV Learning,

Mark Polk
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